The invention relates to a method and an apparatus for the conditioning of phosphoric acid and in particular of mixtures containing phosphoric acid.
Phosphoric acid and mixtures of phosphoric acid and other acids such as sulphuric acid are formed in large quantities in the processing of metallic surfaces.
Metal surfaces are often cleaned, pickled, electro-polished and decontaminated by means of phosphoric acid in its pure form or in the form of a mixture with other mineral acids, e.g. sulphuric acid. For this purpose, it is employed both concentrated as well as in any degree of dilution, both chemically (currentless) as well as electrochemically with the application of current. The objective of the processing is the cleaning of the metal surfaces by removing contaminations or contaminated material layers. To a large extent, the removed material and the contaminants contained therein are dissolved and concentrated in the acid. After processing, the metal surfaces are cleaned from the adhering acid residues by rinsing under water and dried. The resulting rinsing water contains residues of the acids as well as metal ions and contaminants dissolved therein.
With an increasing metal content, the acids and acid mixtures undergo a decreasing effectiveness and must therefore be replaced completely or partially by fresh acid. The thus occurring waste acid and the rinsing water must be conditioned in such a manner that they can be disposed of in an environmentally harmless way. The dissolved heavy metals, in particular, must be converted to solids which are suitable for deposition. Such solids should be neutral to slightly alkaline and must not, or may only slightly be, water soluble.
According to the state of the art the conditioning or phosphoric acid and mixtures of phosphoric acid, which are contaminated as described is performed by dilution with water followed by a neutralisation by means of bases, in particular calcium hydroxide, and by precipitation of the formed salts and metal hydroxides. Subsequently, the solids are separated from the water by sedimentation and filtration, thickened and deposited in special waste dumps or sites. The remaining water contains, in a reduced concentration, the contaminants which were originally included in the acid and is discharged to the environment, provided the contaminant concentration does not exceed the currently valid limits, or is recycled to be partially reused. In most cases, however, the total of the waste solids to be deposited represent a multiple of the acid to be conditioned.
Another possibility for the regeneration of mixtures containing phosphoric acid is the recovery of the phosphoric acid by liquidxe2x80x94liquid extraction and its subsequent reuse. This, however, still leaves the problem of an environmentally compatible conditioning of the remaining residual and waste materials. Moreover, this method necessitates sophisticated and expensive equipment.
In the treatment of radioactively contaminated metal surfaces by means of phosphoric acid and phosphoric acid mixtures the above mentioned methods have serious drawbacks because the acids and the rinsing water themselves which are used in the conditioning will subsequently be radioactively contaminated.
It is obvious that the radioactive contaminants contained in the acids and rinsing water must not be discharged to the environment in a free condition, but must be completely transferred to solids and to an immobilized condition for disposal in a terminal store for radioactive waste. This means that in practice these waste materials must be expensively conditioned in a separate process prior to storage by encasing them in concrete or in bitumen, fixing them in ion exchangers or encasing them in glass, which again results in a considerable volume increase. The available space for the deposition of radioactive waste, however, is limited and the associated costs are extremely high.
The problems with respect to a conditioning method which is capable of meeting the relevant requirements of the waste liquids as they occur in the electrochemical or chemical decontamination of metal surfaces by means of phosphoric acid or its mixtures could yet not be solved satisfactorily. For this reason, chemical and electrochemical decontamination methods by means of phosphoric acid or its mixtures are employed very rarely despite of their high effectiveness and simple handling.
From DE 39 08 125 A1 a method is known, wherein residues from industrial manufacturing processes and in particular residues from combustion plants are solidified. The solidified products can be stored in dump sites or used as construction materials. This documents states that a mixture from alkali silicate and alkali aluminate can be used as a binding agent for the purpose of solidifying the residues, which is added in a solid condition or, if required, as an aqueous solution. According to all examples, the residues concerned are substantially dusts (i.e. flue dust) from combustion plants.
The object of the present invention is to provide a method for the direct and complete transformation of the waste acids and rinsing water including any contaminants contained therein whereby solids suitable for deposition are formed so that compared to the state of the art a significant increase of the volume of the waste acid etc. is prevented and no particularly sophisticated means and equipment are necessary which themselves can become contaminated.
According to the invention this object is solved by a method, wherein aluminum containing alkaline solutions of a quantity sufficient for the formation of solids are added to the mixtures containing phosphoric acid, as they are obtained by the processing of metal surfaces. Surprisingly, it was found that the obtained solids are essentially water insoluble and can therefore be readily deposited in disposal sites.